A SiC semiconductor device having a trench gate type vertical power device is well known. Specifically, the power device made of SiC has high breakdown electric field strength. Since the SiC material has high breakdown electric field strength, the power device made of SiC can control large current. Accordingly, the power device made of SiC is suitably used for motor control in a hybrid vehicle.
When a large current flows through the power device, a channel density should be increased. In a silicon transistor, a trench gate type vertical power MOSFET is used. The structure in the silicon based power MOSFET may be used for the SiC power device. However, in this case, there are the following difficulties.
The breakdown electric field strength of the SiC material is ten times larger than that of the Si material. Accordingly, a voltage to be applied to the SiC material is ten times larger than a voltage to be applied to the Si material. The voltage is also applied to a gate insulation film disposed in a surface portion in a trench of the SiC material, and is ten times larger than that of the Si material. Accordingly, the gate insulation film may be broken down at a corner of the trench gate.
In JP-A-2001-267570, when a trench gate type MOSFET is manufactured, a P conductive type dopant is implanted just after a trench is formed. Thus, a P conductive type bottom layer is formed on a bottom of the trench gate.
However, an on-state resistance is changed in accordance with a distance from a bottom of a P conductive type base region to the P conductive type bottom layer. The inventors have studied about a trench gate type MOSFET as a related art. FIG. 11 shows a SiC semiconductor device having a P conductive type bottom layer 4 as a simulation model according to a related art. FIG. 12 shows a simulation result of a relationship between an on-state resistance and a protrusion amount L. The protrusion amount L is defined as a distance from a bottom of a P conductive type base region 3 in the SiC semiconductor device to the P conductive type bottom layer 4. The protrusion amount L is equal to a distance between the bottom of the base region 3 and the bottom of the trench 5.
As shown in FIG. 12, when the protrusion amount L is sufficiently large, the on state resistance becomes constant. When the protrusion amount L is small, the on state resistance increases. Specifically, when the protrusion amount L is smaller than 0.2 μm, the on state resistance rapidly increases. When the protrusion amount L is narrow, a distance between depletion layers extending from the P conductive type base region 3 and the P conductive type bottom layer 4 to the N conductive type channel layer 15 and the N conductive type drift layer 2 becomes narrow. Accordingly, as the protrusion amount L is smaller, the on state resistance becomes larger.
Thus, it is required for a SiC semiconductor device having a P conductive type bottom layer on a bottom of a trench to reduce a on state resistance.